It is well known that the data which is determined and recorded by tachographs and the like devices, which are installed in motor vehicles, is valuable insofar as it serves to clarify traffic accidents, to determine the type and extent of the use of a vehicle, and to verify the work times and the rest periods for the driver who is employing the vehicle. This necessarily entails the risk that the recordings of these devices may be altered for fraudulent purposes or that attempts may be made to prevent these recordings. On the other hand, there is also the problem of verifying that the power supply of the respective devices, or the associated signal transmission, may have been tampered with. There are many opportunities for tampering, especially since the speed transmitter is externally situated and is connected with the respective devices via electrical lines and plug-in connections.
In order to verify interruptions in the operating voltage, which result in a compression of the driving time, and which may be effected, for instance, by means of a flasher or a windshield wiper interval timer, so as not to be recognized immediately as tampering, it is known to provide a storage circuit, which records a mark, which is atypical of driving, e.g. a radial line over the entire speed recording field in conventional tachographs, when the operating voltage is switched on again.
Further, it is also known to effect the signal transmission twice and to transmit an inverted signal sequence of one and the same speed transmitter via a second signal line. An Exclusive OR gate, located on the device side, serves as an evaluating or recognition circuit which, in the case of a tachograph recording on diagram charts, controls the speed recording member during a disruption in the signal transmission, in such a manner, that it is deflected in a defined manner and records a bar chart if the disturbance persists.
This known device may verify whether or not the speed transmitter is completely severed, whether one of the transmitter lines is interrupted or short-circuited with other lines, or whether the power supply of the transmitter is interrupted. To this extent, this device also offers a relatively reliable self-monitoring of the speed transmitter, including the signal lines and contacts.
In spite of the above, however, there are known methods of tampering with these devices which cannot be detected by the devices described above since they meet the conditions of phase inversion. In other words, they simulate an orderly operation of transmitter and signal transmission. In this manner, the work time codes which are applicable to commercial transportation, and which are often ignored in this highly competitive branch of industry, may be circumvented. In particular, adherence to prescribed rest periods may be falsified or, in the case of taxis or rental cars, illegal trips may be made without any mileage record. Another known practice is the simulation of a speed chart, which makes it possible to exceed legally prescribed speed limits without detection.